How do I prevent and treat black algae in my concrete pool?

Keep chlorine between 1-3 ppm, brush walls and floor weekly, and shock the pool every seven days during swim season. Black algae roots into concrete and forms a waxy layer chlorine can’t penetrate alone, so use a stiff stainless steel brush to break that barrier. Test water weekly, keep pH between 7.2-7.6, and if you see tiny black spots, brush aggressively and apply a 2x dose of shock that day. Skip cheap algaecides — they rarely work on black algae.